Conventionally, an insulated electric wire has been manufactured as follows. For example, a conductor having a circular cross-section passes through a cassette roller die (CRD) equipped with a pair of rollers to be wire-drawn to have a flat cross-section. Then, the conductor passes through an annealing furnace to remove distortions occurred in the wire-drawing process, so that the conductor is softened. Consecutively, the conductor is coated with enamel varnish and passes through a baking furnace to form an enamel-baking layer on the conductor. The resultant insulated electric wire having a flat cross-section is wound. One of these techniques is disclosed in Patent document 1.
In recent years, electrical devices, industrial motors, automobile driving motors and the like are made to be energy-saving, and miniaturized with high performance. Accordingly, an attempt has been made to control the motors through an inverter. Therefore, the insulated electric wire used in the motors tends to be exposed to environments where a corona discharge may occur (a discharge caused by a non-uniform electrical field occurring around a sharp electrode; also known as a local breakage discharge). In order to prevent the corona discharge from occurring in the insulated electric wire, it is known as being effective to increase a thickness of the enamel-baking layer baked on the conductor of the insulated electric wire (refer to Paschen's law). However, since the enamel varnish is expensive, the thicker insulation layer leads to higher production cost.
Therefore, the present applicant has developed an insulated electric wire D2 as illustrated in FIG. 3 (see Patent document 2). That is, in the insulated electric wire D2 as illustrated in FIG. 3, a primary coating layer B including an enamel coating layer B1 is formed on an outer side of the conductor A to form an electric wire D1 (hereafter, referred to as a primary coated electric wire D1). A resin (hereinafter referred to as an extrusion resin) is extrusion-coated (or extruded) on the outer side of the primary coating layer B to form a secondary coating layer C. Accordingly, even when a less expensive extrusion resin is used, it is possible to prevent the corona discharge. In order to obtain the insulated electric wire D2 as structured above, Patent document 2 discloses a technique where the extrusion is carried out with an extrusion resin heated up to a desired temperature.
Patent document 3 discloses a technique, in which when an extrusion resin of polyetheretherketone (PEEK) is formed on a surface of a conductor to form an insulated electric wire, the conductor is pre-heated to suppress reduction in a resin temperature, and an insulation coat is formed on a surface of the conductor, thereby making it possible to eliminate a process of pre-heating the conductor.
Patent document 1: Japanese Patent No. 3604337
Patent document 2: Japanese Patent Laid-Open Publication No. Hei 2005-203334
Patent document 3: Japanese Utility Model Laid-Open Publication No. Sho 58-37617.